1. Field of the Invention
The present invention relates to a fixing apparatus provided in image forming apparatuses such as electrophotographic printers and copying machines, more specifically, a releasing agent supplying apparatus for preventing the offset phenomenon on rotating members used for pressure-thermal fixing of images, and a fixing apparatus equipped with such a releasing agent supplying apparatus.
2. Description of the Related Art
An image forming apparatus such as a printer and a copying machine using the electrophotographic process is typically equipped with a fixing apparatus for fixing unfixed images held on a sheet of paper as a recording medium. There are several kinds of fixing processes. The pressure-thermal fixing is one of the most popular processes. The fixing apparatuses based on the pressure-thermal fixing process can be divided into the belt type and the thermal roller type. For example, a belt type fixing apparatus was disclosed by JP-A-6-318001.
In the fixing apparatus based on the pressure-thermal fixing process, a surface of the sheet that holds the toner makes a direct contact with the outer surface of a rotating member such as a fixing belt or a fixing roller. As a result, a portion of the unfixed toner image can readily be transferred to the rotating member.
Consequently, it causes the offset phenomenon or the problem that the toner adhered to the rotating member can be transferred again to the trailing edge of the sheet to smear said sheet or the next sheet to be sent in, thus smearing said sheet. Therefore, a device has been introduced to coat (supply) the rotating member with a releasing agent that improves the characteristic of releasing the toner from the rotating member. Silicone oil (hereinafter called simply "the oil") is commonly used as the releasing agent.
The conventional oil coating apparatus (releasing agent supplying apparatus) typically has an oil coating roller that coats the rotating member with the oil held therein. Such an oil coating roller typically has a cylinder the outer periphery of which is provided with a plurality of small holes and a porous elastic layer such as a sponge layer that covers the outer periphery of the cylinder to contact with the rotating member under pressure. The oil coating roller is driven by the rotating member and supplies the oil held inside the cylinder through the porous elastic layer to the outer periphery of the rotating member.
In the conventional constitution, the amount of oil coated per sheet does not remain constant from the start of the uses of an oil coating roller until it is used up and the roller is replaced with a new roller, but rather it is delivered more in the early period of the life of the roller and reduces with time towards the end. As a result, towards the end of the oil coating roller's life, the offset phenomenon is likely to occur because the amount of oil coated per sheet is not enough. On the other hand, if the lower limit of the oil coating amount is simply raised in order to prevent the offset in the later stage of the oil coating roller's life, the oil coating amount in the early period becomes too much and may cause oil stains and image noises such as whiting of the image. In order to resolve this problem, it is conceivable to provide a blade to wipe out excessive oil on the rotating member in such a way as to be able to contact under pressure or to back away at will. However, such a provision of a movable blade makes the structure of the fixing apparatus more complicated and requires a complex blade motion control.
In the belt type fixing apparatus disclosed by JP-A-6-318001, the sheet holding the unfixed toner is transferred in such a way as to face against the fixing belt. Therefore, the unfixed toner on the sheet is preheated to a certain degree by the radiant heat from the fixing belt before the fixing belt and the pressure roller arrive at the nip area where they contact each other under pressure. Thus, in case of the belt type fixing apparatus, the temperature of the fixing belt at the nip area can be set lower than in the case of the roller fixing type, making it possible to set the amount of oil coated to about one tenth of that of the roller type fixing apparatus.
In order to take advantage of the belt type fixing apparatus, it is desired to develop the oil coating roller that is capable of coating a smaller amount of oil (e.g., 0.1-10 mg per A4 size sheet) in a stable manner throughout the entire roller life span, from the start to finish.